Carbon fiber composite parts comprise carbon fiber cloth impregnated with a matrix polymer such as a suitable epoxy resin composition. While epoxy resins are commonly used matrix polymers, other polymers such as polyester resins, vinyl ester resins, polypropylene resins, or polyamide resins are also suitable. In low volume production of a given part, a thermosetting composite article is usually produced by layering sheets of carbon fiber cloth onto a mold cavity surface that defines the shape of the desired product. The alignment and weave of the carbon fibers in the cloth may be important in the mechanical performance of the resulting product. The layered sheets of carbon fiber cloth are carefully infiltrated and impregnated with uncured liquid polymer material. The liquid material is carefully flowed around each fiber or strand of the cloth to form a fiber and liquid composite with minimal air or gas voids. The liquid is uncured precursor material for the thermosetting polymeric material that will constitute a solid matrix around each fiber of the carbon cloth or other fibrous carbon reinforcing material. Impregnation with the liquid polymer precursor may be accomplished before or after the layered sheets are placed in the mold. There the layers are compacted into a desired shape and the liquid polymeric precursor matrix is heated and cured to a solid matrix. The cured polymer-carbon fiber composite product is light in weight and very strong.
For low volume production liquid polymer precursor (sometimes called “resin”) impregnated carbon cloth sheets (called “prepreg”) may be laid up by hand on a single-sided tool. Prepreg sheets about 0.2 mm thick and comprising, for example, 40 weight percent liquid epoxy resin precursor and 60 weight percent of a single layer of carbon cloth are cut to a desired shape and laid up in multiple layers on a mold surface to obtain a part shape. The assembly is then placed in a vacuum and compressed and heated to promote resin flow to eliminate voids in the molding. Heating at a suitable temperature cures the epoxy resin (or other resin material) to form a strong carbon composite product characterized by reinforcing layers of carbon fiber cloth in a polymer matrix. This composite is very strong and its constituents are of relatively low specific gravity. Compared to other candidate materials of construction, carbon fiber composites provide a unique combination of stiffness and low weight.
High value carbon fiber composite parts are used, for example, in aerospace applications, high performance sail boats and bicycles, and in racing vehicles or unique light-weight super cars. These “high value” applications permit the use of high labor cost, hand lay-up molding practices. And the low production volumes permit the use of molding tool materials that do not provide hard surfaces for resisting wear from the abrasive composite materials. The high volume production, for example, of automobile body panels requires molding tools in which many identical panels may be successively formed to high dimensional accuracy.
Invar tooling has been used for molding precision carbon fiber composite panels with complex geometry in both aircraft and automotive industries. The alloy, Invar-36, consists, by weight, of 36% nickel and 64% iron. It became the preferred tooling material because of its unique thermal expansion behavior: a coefficient of linear thermal expansion (CLTE) of
2×10−6 per ° C. in the temperature range of 25° C.-150° C., very close to that of the carbon fiber composites. The tooling material is considered to be too soft for production of a large number of composite parts. The hardness of Invar-36 (80 HRB) is significantly lower than that of the P20 tool steel (50 HRC), suggesting that Invar tooling will not be sufficiently wear-durable by automotive standards even for low volume production.
It is an object of this invention to provide a suitable tool material for molding liquid polymer precursor infiltrated, carbon fiber composite parts in high volumes where the tool surface is continually and repeatedly exposed to abrasive contact with the composite material. An example of such a manufacturing situation is the molding of automotive body panels at automotive production volumes.